I mean fireball is pretty obvious, but only when you cast it.

Ioun stones are very obvious since they orbit your head. That pretty much screams "Magic!"

But I think the Incarnate Avatar soulmeld (tied with some other soulmelds, like Girallon arms)) might be the winner. You have another body superimposed on your own all day, and it is the body of an outsider. I think others can still see your real body underneath, unless the soulmeld is bound to a chakra.

So what other candidates are there for "I am USING MAGIC DOOODZ!!!!" least subtle magic in 3.5?